The effects of maternal or fetal hypophysectomy on the growth and development of endocrine organs of fetal rhesus monkeys was studied. After maternal hypophysectomy, fetal adrenals were enlarged 3 fold; whereas fetal ovaries and thyroids were not affected strikingly. In contrast, fetal hypophysectomy led to markedly diminished growth of ovaries, adrenals and thyroids. We conclude that the fetal endocrine organ - hypophysial axes of the rhesus monkey are dynamic systems by mid-gestation; perhaps able to monitor and integrate the fetal compartment with the entire hormonal milieu of pregnancy. Survival of gametes within the fetal ovaries of the primates is dependent primarily on secretions of the fetal pituitary. Likewise, growth of seminiferous tubules within fetal testes depends very much on hormone output from the fetal hypophysis.